Polyurethane foams are widely used in automotive, housing and other industries. The foams are produced through the reaction of a polyisocyanate with a polyol in the presence of various additives. One class of additives which is particularly effective as blowing agents is the chlorofluorocarbons (CFCs). CFCs vaporize as a result of the reaction exotherm during polymerization and cause the polymerizing mass to form a foam. However, the discovery that CFCs deplete ozone in the stratosphere has resulted in mandates for restricting CFC use. Therefore, more efforts have gone into the development of alternatives to CFCs for forming urethane foams and water blowing has emerged as an important alternative. In this method, blowing occurs from carbon dioxide generated by the reaction of water with the polyisocyanate. Foams can be formed by a one-shot method or by formation of a prepolymer and subsequent reaction of the prepolymer with water in the presence of a catalyst to form the foam. Regardless of the method, a balance is needed between reaction of the isocyanate and the polyol (gelling) and the reaction of the isocyanate with water (blowing) in order to produce a polyurethane foam in which the cells are relatively uniform and the foam has specific properties depending on the anticipated application; for example, rigid foams, semi-rigid foams, and flexible foams.
The ability of the catalyst to selectively promote either blowing or gelling is an important consideration in selecting a catalyst for the production of a polyurethane foam with specific properties. If a catalyst promotes the blowing reaction to too high a degree, carbon dioxide will be evolved before sufficient reaction of isocyanate with polyol has occurred. The carbon dioxide will bubble out of the formulation, resulting in collapse of the foam and production of a poor quality foam. At the opposite extreme, if a catalyst promotes the gelling reaction too strongly, a substantial portion of the carbon dioxide will be evolved after a significant degree of polymerization has occurred. Again, a poor quality foam is produced; characterized by high density, broken or poorly defined cells, or other undesirable features. Frequently, a gelling catalyst and a blowing catalyst are used together to achieve the desired balance of gelling and blowing in the foam.
Tertiary amine catalysts have been used to accelerate both blowing (reaction of water with isocyanate to generate carbon dioxide) and gelling (reaction of polyol with isocyanate) and have been shown to be effective in balancing the blowing and gelling reactions to produce a desirable product. However, tertiary amines generally have offensive odors and many are highly volatile due to low molecular weight. Release of tertiary amines during foam processing may present significant safety and toxicity problems, and release of residual amines from consumer products is generally undesirable. Reduction in the amount of tertiary amine catalysts used in the production of polyurethane foams is therefore desirable.
The following references disclose a variety of compounds which can be used alone or in combination with tertiary amines to accelerate curing of polyurethane foams and/or produce polyurethane foams with enhanced properties.
U.S. Pat. No. 3,073,788 (Hostettler et al., 1963) discloses the production of foamed polymers from isocyanate-modified alkylene oxide addition products of glycosides in which a variety of compounds are used to catalyze the foaming and crosslinking reaction; for example inorganic and organic bases such as sodium hydroxide, tertiary amines and phosphines, various metal compounds, such as titanium tetrachloride, bis(cyclopentadienyl) titanium difluoride, and a wide variety of organic tin compounds.
U.S. Pat. No. 3,201,358 (Hostettler et al., 1965) discloses a method of preparing polyurethane foams using a catalyst of antimony trichloride, titanium tetrachloride, or dioctyl lead dichloride in combination with a tertiary amine in which the nitrogen is bonded to three aliphatic and cycloaliphatic carbon atoms.
U.S. Pat. No. 3,222,305 (Lanham, 1965) discloses a wide variety of catalysts for accelerating the reactive hydrogen/isocyanate reaction in producing flame-resistant urethane foams. Examples of disclosed catalysts are tertiary amines; tertiary phosphines; strong bases; acidic metal salts; chelates of various metals, such as magnesium, bismuth, and titanium; alcoholates and phenolates of various metals; and organometallic compounds. Dibutyltin dilaurate alone or together with N,N,N',N'-tetramethyl-1,3-butanediamine was used in the examples.
U.S. Pat. No. 3,450,648 (Windemuth et al, 1969) discloses a process for making cellular polyurethane plastics in the presence of a catalyst which is a mixture of a tertiary amine and one of eleven specific non-basic metal compounds; for example, TiCl.sub.2 (C.sub.5 H.sub.7 O.sub.2).sub.2, titanium tetrabutylate, and titanium dichloride diethyl. At levels of about 0.00001 to 0.1% weight of metal based on the total weight of the reactants, the non-basic metal compounds were reported to be useful in producing foamed plastics with improved tensile strength and no cracking.
Following are references which describe the use of Group 3 and 4 metal compounds for the preparation of polyurethanes, without the formation of bubbles or excess foam, even in the presence of water.
U.S. Pat. No. 3,673,159 (Dijkhuizen et al., 1972) discloses the use of organic zirconium compounds, specifically zirconium alcoholates, as catalysts for the preparation of unfoamed polyurethane from the reaction of isocyanates with polyols. If the polyol component contains trace water, the catalyst is added prior to the isocyanate to remove the water.
U.S. Pat. No. 4,264,486 (McLaughlin, 1981) and U.S. Pat. No. 4,374,968 (McLaughlin, 1983) disclose a method of forming an isocyanate polymer with little foam, in the presence of water, by using a catalytic mixture containing a Group 3 or 4 metal compound, such as dibutyl tin diacetate and tributyl titanate. Since these catalysts do not catalyze the reaction between the isocyanate and water, the gelation times are extended and little foam is produced.